Luteinizing hormone releasing factor in rat hypophysial portal blood collected during electrical stimulation of the hypothalamus.

1. Ovulation was induced in Nembutal-blocked pro-oestrous rats by electrical stimulation of the hypothalamus.2. The same type of electrical stimulation was applied during the collection of hypophysial portal blood.3. Pooled hypophysial portal plasma from donors in pro-oestrus, oestrus and met-oestrus was assayed for ovarian ascorbic acid depleting (OAAD) activity.4. Electrical stimulation of the hypothalamus increased the OAAD activity, believed to be due to luteinizing hormone releasing factor (LRF), in pro-oestrus and met-oestrus, but not in oestrus.5. It is concluded that the hypothalamic nerve fibres responsible for releasing LRF into the hypophysial portal vessels are depleted of their store of this releasing factor, or are refractory to electrical stimulation, during oestrus.